refactor(frontend): extract shared modules, thin views, delete dead code

Shared utilities:
- Add src/utils/formatters.js — formatDate, formatSize, providerColor,
  providerBg, providerLabel; all components import from here, no inline duplicates
- Add src/components/ui/TreeItem.vue — generic expand/collapse tree node;
  FolderTreeItem, CloudFolderTreeItem, CloudProviderTreeItem now wrap it
- Add src/components/storage/StorageBrowser.vue — unified file browser grid
  used by both FileManagerView and CloudFolderView

View refactor (thin data-providers):
- FileManagerView.vue: stripped to props + event wiring; all layout moved to StorageBrowser
- CloudFolderView.vue: same treatment — feeds props into StorageBrowser
- All tree sidebar components delegate expand/collapse to TreeItem.vue

Dead code removed:
- Delete HomeView.vue — no active route, replaced by FileManagerView
- Delete FolderView.vue — no active route, logic merged into FileManagerView

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/components/admin/AdminUsersTab.vue

@@ -268,6 +268,7 @@
 
 <script setup>
 import { ref, reactive, onMounted } from 'vue'
+import { formatDate } from '../../utils/formatters.js'
 import * as api from '../../api/client.js'
 
 const users = ref([])
@@ -464,14 +465,6 @@ async function resetPassword(id) {
   }
 }
 
-function formatDate(iso) {
-  if (!iso) return '—'
-  try {
-    return new Date(iso).toLocaleDateString('en-US', { year: 'numeric', month: 'short', day: 'numeric' })
-  } catch {
-    return iso
-  }
-}
 
 onMounted(async () => {
   loading.value = true